It is alleged that traders used online chat rooms to collude in the fixing of benchmark prices.

Scrutiny is focused on activity around London's 4 p.m. currency fix, a 60-second window where key exchange rates are set. These prices are used as reference rates for trillions of dollars of investment and trade globally.

Britain's financial regulator, the Financial Conduct Authority, launched its probe last October and the Bank of England in March appointed barrister Anthony Grabiner to examine whether any of its officials were involved in forex rigging.

The United States Department of Justice opened a criminal probe into the matter last October.